


Eternal

by TellTaleKael



Category: Undertale (Video Game)
Genre: All The Bad Sanses Have Chronic Illnesses, Arthritis, Chronic Fatigue, Chronic Illness, Chronic Pain, M/M, Magical Illness, Nightmare/Killer/Dust/Horror/Error/Cross, bad sans poly - Freeform, fibromyalgia, migraines
Language: English
Status: In-Progress
Published: 2021-03-07
Updated: 2021-03-07
Packaged: 2021-03-12 21:21:07
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 1,899
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29890707
Author URL: https://archiveofourown.org/users/TellTaleKael/pseuds/TellTaleKael
Summary: Nightmare never expected to collect a bunch of skeletons with chronic illnesses, but fate it a funny thing.
Relationships: Sans/Sans (Undertale)
Comments: 29
Kudos: 119





	1. Nightmare’s Spine

**Author's Note:**

> This fic is a commission for [TheOrdinaryOli](https://twitter.com/theordinaryoli?s=21) over on Twitter. There will probably be five or six chapters, and I’m trying to get one chapter a day posted, so look forward to it!

By the time Nightmare called an end to their sparring session, his spine was screaming at him for pushing things too far. Killer was admittedly quick on his feet, and good at misdirection, feinting and slipping around with an almost dancer-like grace. Several times, Nightmare had been forced to grab the other with his tentacles, using their strength to toss Killer aside. Bearing the weight had hurt, but it had been worth it to feel the surprise coming off the odd skeleton. Nightmare wasn’t feared across the multiverse for nothing, and if Killer was going to work under him, he had to realize that for himself. 

“You’re reckless,” Nightmare said when Killer fell in step beside him. They had been out in the courtyard for the match, so as to not break anything. Killer shrugged.

“still almost got close enough to land a hit.”

“Almost being the key word there.” Nightmare scoffed. “You’ll have to learn to work alongside others.”

“mm, guess i’m not enough to satisfy you?”

Nightmare just barely managed to not roll his eyelight. “I’m not deigning that with a response.” So far, Killer’s MO seemed to be casual flirting at the drop of one of his knives, even if Nightmare’s presence still made him uneasy. It was best to just write him off as a masochist and be done with the whole situation.

They stepped into the castle, leaving the dim glow of twilight behind for the slightly less dim glow of candles illuminating the corridors.

“what now?” Killer asked. Nightmare glanced at him before continuing down the hall.

“Now I’m retiring to my quarters. I do not care what you do as long as nothing turns up broken.” With that, he swept away, leaving Killer behind.

Killer was the first, the first Sans that Nightmare had managed to bring over to his cause. He had others in mind, of course, sending feelers out to the worst of the worst universes to find those skeletons broken enough to fall under his command. It was almost time to approach the second, but until then, it was just Nightmare and Killer. Having someone around who didn’t immediately flee at the sight of him was... odd, after so many years alone. 

The door to Nightmare’s quarters closed behind him, and immediately his tentacles sagged down to the floor. He closed his socket, letting out a deep, shaky breath. The bed was just in the other room, but the distance seemed insurmountable, now that he had allowed himself to feel the pain that had been nagging at him all afternoon.

His original skeleton body just had not been equipped for extra appendages, and certainly not ones so long and thick and heavy. To keep up with the change, his spine grew thicker, vertebrae fusing together to make up for the added weight he could no longer banish. The bones rubbed together uncomfortably, sending grinding pain across his senses with every movement. If anyone knew of his weakness... well. He would kill anyone who found out immediately, before they used that knowledge against him. He had plenty of practice hiding his pain away after all of these years.

Pushing away from the door, he managed to drag himself into the bedroom. The mattress on his best was the softest he could find, allowing him to sink into it. He curled up, still dressed, letting the bed cradle him in a position that allowed his spine to finally relax. The ache that radiated from the middle of his back was sharp, but he knew if he remained still as long as possible it wouldn’t get worse. The corruption that coated his body rippled, before settling down. He snatched one of the multitude of pillows surrounding him, hugging it to his chest.

He just needed a little time to compose himself, that was all. Then he would be able to replace the mask that hid the nonstop pain he had no way of relieving. No one had to know. He was _fine_.


	2. Killer’s Head

Nightmare was almost finished reading the report on the universe casually dubbed ‘Horrortale’, and there hadn’t been a single interruption the whole time he had been reading. He swung his arms forward in front of him, taking a moment to stretch his spine. Usually, when he had been reading for a while, Killer would appear, as if he had a radar for when Nightmare was busy working. Sometimes he would simply perch somewhere in the office and thumb through a book, but most the time he was loud and obnoxious in one way or another, chattering away or throwing and catching his knives in increasingly risky maneuvers until Nightmare could no longer focus on his task. This was the first time he had managed to finish anything without so much as a peep from Killer.

In fact, now that he thought about it, Nightmare realized that he hadn’t seen the other all day. Folding the report, he sat back in his chair, seeking out Killer by his emotions.

_ Pain. Agony. Frustration. _

Nightmare took a moment to soak in the negativity, before standing. The emotions were different than what he usually felt from Killer, sharper and more present. What could have caused it, though?

Easily enough, Killer was in his room. Nightmare knocked on the door, intrigued to feel the pain flare brightly in the room beyond. He closed his hand around the doorknob, finding it unlocked. He slowly pushed it open.

The room beyond was completely dark. The candles had been snuffed out, a blanket draped over the window, and even what appeared to be Killer’s jacket stuffed in the crack at the bottom of the door, all with the clear intent of blocking out the light.

In the middle of the bed was a shaking form, wrapped in layers of sheets. One of the lumps turned, and Nightmare realized it was Killer’s skull turning towards him. The pain flared again, and Killer let out a hissing breath. 

“no light,” came a raspy voice from under the blanket. “please.” There was a hint of pleading in Killer’s usually confident tone. Nightmare allowed the door to close behind him, casting the room in to almost complete darkness again. Relief emanated from Killer. 

“What’s going on?” Nightmare asked, voice dropping low when Killer reacted with pain once again.

“migraine. happens. no big deal.” There was a faint rustling as Killer tried to get comfortable on the bed. It certainly didn’t seem like no big deal... but Nightmare could understand downplaying pain in an effort to not appear weak. 

“I see,” he whispered. He crossed the room, perching on the edge of the bed, one tentacle seeking out Killer’s skull through the blanket. Killer flinched at the first touch, but he relaxed when Nightmare siphoned off some of the pain. He couldn’t take it all away, but anything would help, he figured.

“you don’t have to be here,” Killer rasped. “it’ll go away. doesn’t last long.”

Nightmare allowed his tentacle to rest lightly on Killer’s skull. “shut up and recover,” he said. “i’m staying.”

Killer moved in what Nightmare guessed was a shrug, before stilling. The rise and fall of the lump that signified his breathing was steady and even for a long time, before finally slowing down. Nightmare waited, feeling the pain fade as Killer drifted off to sleep. He didn’t know much about migraines.

Perhaps he needed to do some research. 

***

It was quite a while before Killer experienced another migraine. Nightmare had already contacted the next Sans he was after, and had found several other candidates as well. In that time, he continued training Killer, directing him to hone his skills until they were as sharp as the knives he wielded. Killer was more brutally efficient in his attacks, and more open with desire to serve Nightmare... in several ways. It felt as if he were trying to make up for the moment of weakness he had shown when Nightmare found him in that darkened room.

Nightmare didn’t see the condition as lessening Killer’s usefulness. He knew he had his own troubles, yet he was still the scourge of the multiverse, after all. Killer was still vicious and loyal.

They were eating when the flash of pain flared in Killer. He froze with his fork halfway to his mouth, grin becoming strained around the edges. He put his fork down, trying to appear casual. “i think i’m full, boss. permission to hit the hay early?”

“No,” Nightmare said, lowering his own utensils to the table. Distress emanated from Killer as Nightmare stood. “Follow me,” he commanded, before sweeping away. He didn’t have to reach for Killer’s emotions to know that the other followed. He always followed.

The room was just down the hall from Killer’s. Nightmare opened the door, revealing a white pod, split open from the middle to reveal a shallow pool of water. He had nicked it from one of the science Sanses- universes where the Sans was still a young, eager scientist, without the nihilism and exhaustion his older counterparts commonly had. Turning to a confused Killer, Nightmare commanded, “Strip.”

“if you wanted to see me naked, all you had to do was ask,” Killer teased, voice slightly strained as he shrugged his jacket off. By the time he was bareboned, the pain had grown. Nightmare couldn’t help but feed off of it for a moment, before shaking his head. “look,” Killer said, “if you want me to summon it’s going to take some work. not that you aren’t sexy, but i’ve got a bit of a headache.”

“We’re not having sex,” Nightmare ground out, pinching his nasal aperture. 

“we’re not?” There was genuine disappointment in Killer’s tone.

“No. Do you know what this is?”

Killer looked at the pod. “some science fiction bed?”

“Get in and lie down,” Nightmare demanded. Killer did as asked, resting in the shallow, salty water. 

“weird.”

“I’m closing you in now.”

“why?” Killer’s hand shot out as if to hold the lid open, even though Nightmare hadn’t moved to shut it yet.

“Killer.” Nightmare waited. Would the other trust him enough to allow him to shut the tank? It would be easy to explain what it was... but that would force him to admit the research he had done, and the planning that had taken place, all to lessen Killer’s pain. 

Slowly, Killer lowered his arm. “okay, boss.”

Nightmare closed the lid carefully, plunging Killer into darkness. The panic that overtook him only lasted several moments, until Nightmare stole it away. He stood by the tank until the pain began to lessen, and in it’s place rose awe. Only then did he cross the room to perch on the single chair he had left in the corner.

Killer remained in the sensory deprivation tank for an hour and a half. Nightmare gently tapped on the lid, before opening it.

“the migraine is gone,” Killer blurted immediately. 

“I know.” Nightmare turned to leave. “Go shower before the salt crystallizes in your joints. This room is yours to use as you will.”

He ignored the burst of fondness that followed him out of the room. A good warrior kept their weapons polished and ready to work, after all. That’s just what he was doing. It had been worth the pain of lifting the tank with his tentacles to get it adjusted just right. 

**Notes for the Chapter:**

> Nightmare, prince of negativity? More like Nightmare, king of denial.
> 
> Also I want to try a flotation tank, they sound amazing.

**Author's Note:**

> The rest of the chapters will be longer, this was just an introduction.


End file.
